1. Field of the Invention
The present invention relates to an air bag system (air bag ECU) which expands an air bag for occupant protection by detecting the occurrence of a vehicle crash and, more particularly, to an air bag system which employs a safing system in determining the occurrence of a crash using an acceleration sensor.
2. Prior Art
Vehicles such as passenger cars have long been equipped with air bag systems that automatically expand air bags to protect occupants, in the event of a vehicle accident, by detecting an impact using electronic acceleration sensors. Such air bag systems employ a so-called safing system in which the determination as to whether to expand or not expand the air bag is made using not only a first channel (main crash determination line) comprising a main electronic acceleration sensor and an electronic circuit but also a second channel (safing line) that operates independently of the first channel in order to ensure safe air bag expansion in case of a malfunctioning of the first channel; more specifically, the system operates such that the air bag is expanded by determining that a vehicle crash has occurred only when the results of the determinations from the two channels match. Since the probability that both the main crash determination line and the safing line make erroneous determinations is extremely low, the above system works sufficiently effectively to prevent the danger of the air bag being erroneously expanded in a non-crash situation due to failure of the acceleration sensor or other electronic systems.
In the prior art, there is also an air bag system equipped with a self-diagnostic function that automatically diagnoses operational faults in various electronic systems in the air bag system, for example, at the time of ignition on, in order to reliably expand or not expand the air bag (for example, refer to Japanese Unexamined Patent Publication Nos. H05-178163, 2001-206191, and 2002-370609).
As these prior art air bag systems employ the safing system and incorporate self-diagnostic functions, electronic control becomes extremely complex in such systems. Accordingly, microcomputers capable of implementing a large number of control functions in software are used. Further, the sating system requires two microcomputers to implement, i.e., a microcomputer for use in the first channel for crash determination and a sub-microcomputer for use in the second channel for safing purposes. If the first and second channels were implemented in one microcomputer, then in the event of runaway of the microcomputer the two channels would become unable to make correct determinations, rendering the sating system meaningless.
FIG. 1 is a diagram showing the basic configuration of a prior art air bag ECU equipped with a safing system. In the figure, reference numeral 100 is the air bag ECU which includes a main electronic acceleration sensor (hereinafter called the G sensor) 101, a main microcomputer 102, a sating G sensor 103, a sub-microcomputer 104, an output circuit 105, and a communication IC 106. Here, the communication IC 106 has the function of providing the crash signals generated by front sensors 107 and satellite sensors 108, the former mounted in the right and left front end positions of the vehicle and the latter in both sides of the vehicle, to the main microcomputer 102 and sub-microcomputer 104 so that further detailed determinations (including the position and severity of the crash) can be made there; however, as the communication IC 106 is not a component that constitutes a feature of the present invention, a detailed description thereof will not be given here.
Reference numeral 109 is an electronic switch which is connected to an internal power supply 112, and which is turned on by an ignition signal from the main microcomputer 102 and supplies a current to an ignition circuit 110 in the output circuit 105 for igniting a squib 111. The output circuit 105 contains a gate circuit 113 which, based on a crash signal supplied from the main microcomputer 102 and a safing signal, generates a signal for activating the ignition circuit 110. The air bag ECU 100, the front sensors 107, the satellite sensors 108, and the squib 111 are located inside the vehicle 500.
In the air bag system of FIG. 1, the main microcomputer 102 analyzes the acceleration signal supplied from the main G sensor 101 and determines whether a vehicle crash has occurred or not; if it is determined that a crash has occurred, the main microcomputer 102 supplies a crash signal to the output circuit 105 and the switch 109. On the other hand, the sub-microcomputer 104 analyzes the acceleration signal supplied from the safing G sensor 103 and determines, on a channel different to that of the main microcomputer 102, whether a vehicle crash has occurred or not; if it is determined that a crash has occurred, the sub-microcomputer 104 supplies a safing signal to the output circuit 105.
The gate circuit 113 in the output circuit 105 outputs an ON signal to the ignition circuit 105 only when both the crash signal from the main microcomputer 102 and the safing signal from the sub-microcomputer 104 are input to it. Accordingly, if the main microcomputer 102, for example, runs out of control and outputs a false crash signal, as the safing signal is not output from the sub-microcomputer 104, the output circuit 101 is not activated, so that the air bag is not expanded. In the system of FIG. 1, erroneous activation of the air bag is prevented, unless both the main microcomputer 102 and the sub-microcomputer 104 run out of control simultaneously.
In the prior art air bag system shown in FIG. 1, the sub-microcomputer must be provided in addition to the main microcomputer in order to construct the above-described safing system. However, microcomputers are expensive items, and the requirement of two such expensive microcomputers has been a major factor that increases the cost of the air bag system. If the functions to be performed by one of the microcomputers can be implemented in a single IC (originally, one IC is provided for the ignition drive circuit), the cost of the air bag system can be substantially reduced; however, it is difficult to implement complex control functions such as crash determination and self-diagnosis in a single IC and, if such an IC is to be achieved, the cost may increase rather than decrease.